A vacuum heat treatment apparatus, which heat-treats a raw material in a crucible to form a desired material, performs the heat treatment in vacuum environment to prevent contamination from surroundings.
In such a vacuum heat treatment apparatus, the raw material is heated by a heater installed in the thermal insulator placed in a chamber maintained in a vacuum state. The vacuum heat treatment apparatus may comprise a thermocouple in order to measure the temperatures at each part of the vacuum heat treatment apparatus while heating the raw material.
At this time, in order to prevent the thermocouple from corroding or cracking by a reaction gas, a protective tube generally surrounds an outer surface of the thermocouple. However, since a conventional protective tube is formed of molybdenum (Mo) or aluminum oxide (Al2O3), the conventional protective tube formed of the molybdenum (Mo) or the aluminum oxide (Al2O3) material reacts with the reaction gas, so the protective tube may be corroded and the gas-tightness of the protective tube may be degraded. That is, the reaction gas directly exerts an influence upon the thermocouple, so that the life span of the thermocouple may be shortened.